National cross-sectional data show that condom use, contraceptive use, and prevalence of sexually transmitted diseases (STDs) among adolescents vary by age. Prospective data are needed to document that this happens with age and is not due to cohort effects, and to identify factors that account for these patterns. Based on the initial grant period, we are well-positioned to explore determinants of these patterns. We developed a reliable and valid measure of partner-specific perception of risk for sexually transmitted diseases (PRSTD) among adolescents which we showed to be linked to intention to use condoms. These intentions, in turn, predicted future sexual behaviors, especially among adolescents who have more relative power in their sexual relationships. We also established an association between these behaviors, and incidence of biologically-validated STDs. The proposed five year continuation will expand upon the initial project by addressing the following specific aims: 1) determine whether age-related changes within individuals in their PRSTD, patterns of sex partnerships, and condom use are assoicated with changes in incidence of STDs; 2) determine if decrease in condom use and increases in hormonal contraceptive use with age are related to changes in individuals' patterns of sex partnerships and desire to be pregnant; 3) to determine the extent to which the relative power an individual has in a sexual relationship affects condom behavior; and 4) to determine whether the accuracy of adolescents' PRSTD increases with age. In the proposed study, we will follow sexually experienced adolescents, 14-19 years old, over three years. We will collect data (interviews and STD testing) from adolescents whom we have already recruited in the initial period of this project and from adolescents we will recruit during the first year of the proposed project. The cohorts to be studied in the proposed project will include an STD clinic cohort (n=350) and a general adolescent medicine clinic cohort (n=460). We will also recruit and interview main sex partners of a random sample of our subjects. Although these clinic-based samples are not representative of the general adolescent population, data collected from these cohorts complement the data collected from national representative samples. Ultimately, the findings of this study may provide important information which could improve the long-term effectiveness of sexual risk reduction interventions by providing a more dynamic picture of the determinants of adolescent sexual behavior and risk for STDs than previously has been available.